An opaque to semitransparent film comprising a polyolefin film containing a fine inorganic powder or a laminate having such a fine inorganic powder-containing polyolefin film on at least one side thereof, especially a film or laminate in which the fine inorganic powder-containing polyolefin film has been stretched at least uniaxially, is useful as synthetic paper.
Synthetic paper comprising the above-mentioned fine inorganic powder-containing thermoplastic resin film as a base has experienced steadily increasing use over a very board area because of its excellent physical properties such as water resistance, toughness, and surface smoothness. With its increasing utility, a synthetic paper of this type having more excellent antistatic properties and printability in offset printing or printing with ultraviolet-curing inks is in greater demand than ever.
Because polyolefins per se are non-polar and hydrophobic, polyolefin synthetic paper does not always have satisfactory antistatic properties or offset printability, and it is usually subjected to an appropriate surface treatment to improve these properties. The application of a coating agent is among such surface treatments. Known coating agents for synthetic paper include cationic antistatic agents and amphoteric antistatic agents as disclosed in JP-A-50-161478, JP-B-59-27769, and JP-B-2-2910 (corresponding to U.S. Pat. No. 4,420,530) (the terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined published Japanese patent application", respectively).
More specifically, JP-B-2-2910 discloses a water-soluble coating composition comprising (1) a water-soluble polymer which is obtained by preparing a tertiary nitrogen-containing polymer from a monomer represented by the formula CH.sub.2 =CR.sup.1 --COOANR.sup.3 R.sup.4, a monomer represented by the formula CH.sub.2 =CR--COOR.sup.2, and other hydrophobic vinyl monomers, wherein R and R.sup.1 are each a hydrogen atom or a methyl group; R.sup.2 is an alkyl group containing 1 to 18 carbon atoms; R.sup.3 and R.sup.4 are each a hydrogen atom or an alkyl group containing 1 or 2 carbon atoms; and A is an alkylene group containing 2 to 6 carbon atoms and quaternizing the tertiary nitrogen atom of the resulting polymer with a cationizing agent to make the polymer amphoteric, (2) a water-soluble polyaminepolyamide-epichlorohydrin adduct, and (3) a polyethyleneimine compound.
JP-A-53-8380 discloses a conducting agent having an antistatic effect which comprises a polymer comprising a repeating unit represented by the formula: ##STR3## wherein R represents a hydrogen atom or a methyl group; R.sup.1 and R.sup.2 each represent a methyl group or an ethyl group; R.sup.3 and R.sup.4 each represent a methyl group, an ethyl group, --CH.sub.2 CH.sub.2 O--.sub.m H, wherein m is an integer of 1 to 4, or a benzyl group; R.sup.5 represents an alkyl group having 1 to 18 carbon atoms, an alkenyl group or --CH.sub.2 CH.sub.2 O--.sub.p H, wherein p is an integer of 1 to 4; X.sup.- represents a halide ion; A represents a substituted or unsubstituted alkylene group; and n represents an integer usually ranging from 10.sup.1 to 10.sup.4.
Application of these antistatic agents makes the synthetic paper printable by general antistatic offset printing, but the antistatic agents are still insufficient as an antistatic treatment for assuring printability during printing at very high speeds or under severe conditions. In other words, an increase in ion quantity, in an attempt to improve antistatic properties, would make the antistatic agent excessively hydrophilic, making offset printing infeasible. The limit of the ion quantity with weight attached to offset printability has thus been a bar to further improvement in antistatic properties. Therefore, none of the coating agents proposed to date exhibit both satisfactory antistatic properties and printability. On the other hand, where offset printing is performed at high speeds using ultraviolet-curing offset printing inks instead of the conventional solvent evaporation type offset printing inks, printed sheets are forwarded while wet which causes an image defect called white spots.